1. Field of the Invention
The present invention relates generally to a network for wireless communication, and in particular, to a radio-over-fiber (RoF) network in which a central station (CS) is coupled to at least one remote access unit (RAU) using at least one optical fiber.
2. Description of the Related Art
When various wireless communication services, such as second generation (2G), third generation (3G) wireless local area network (WLAN), and portable Internet are provided in a certain area, a plurality of remote access units (RAUs) for supporting the various wireless communication services and a space for the RAUs are required. Thus, schemes for providing various services using a single RAU have been suggested. Using these schemes, the number of RAUs and the number of repeaters can be reduced. Herein, such a repeater amplifies an attenuated signal by being installed between a central station (CS) and an RAU.
Conventional duplexing methods of separating a downstream signal and an upstream signal in a wireless communication network include a frequency division duplexing (FDD) method of assigning different frequencies to downstream and upstream signals and a time division duplexing (TDD) method of assigning different time slots to downstream and upstream signals. To share an RAU, a structure for supporting these different duplexing methods is required.
FIG. 1 illustrates a block diagram of a conventional radio-over-fiber (RoF) network 100. Referring to FIG. 1, the RoF network 100 includes a CS 110 and an RAU 140 coupled to the CS 110 through first and second optical fibers 130 and 135.
The CS 110 includes an electric-to-optical converter (E/O) 120 for converting an electrical signal to an optical signal and an optical-to-electric converter (O/E) 125 for converting an optical signal to an electrical signal.
The E/O 120 is coupled to the first optical fiber 130. The E/O 120 electric-to-optical converts an input first downstream electrical signal SD1 of the TDD method to a first downstream optical signal, electric-to-optical 120 converts an input second downstream electrical signal SD2 of the FDD method to a second downstream optical signal, and outputs the first and second downstream optical signals to the first optical fiber 130.
The O/E 125 is coupled to the second optical fiber 135 and receives a first upstream optical signal of the TDD method and a second upstream optical signal of the FDD method from the second optical fiber 135. The O/E 125 optical-to-electric converts the first upstream optical signal to a first upstream electrical signal SU1 and the second upstream optical signal to a second upstream electrical signal SU2. Herein, different time slots are assigned to the first downstream and first upstream electrical signals SD1 and SU1, and the first downstream and first upstream electrical signals SD1 and SU1 have the same frequency. The frequency of the first downstream and first upstream electrical signals SD1 and SU1, a frequency of the second downstream electrical signal SD2, and a frequency of the second upstream electrical signal SU2 are different from each other.
The RAU 140 includes an O/E 150 for converting an optical signal to an electrical signal, an E/O 175 for converting an electrical signal to an optical signal, a high power amplifier (HPA) 155 for amplifying an input electrical signal with a high gain, a low noise amplifier (LNA) 170 for amplifying an input electrical signal with low noise, a frequency independent circulator (CIR) 160 for performing signal coupling, and an antenna 165.
The O/E 150 is coupled to the first optical fiber 130 and receives the first and second downstream optical signals from the first optical fiber 130. The O/E 150 optical-to-electric converts the first and second downstream optical signals to first and second downstream electrical signals.
One end of the HPA 155 is coupled to the O/E 150, and the other end is coupled to the CIR 160. The HPA 155 amplifies each of the first and second downstream electrical signals input from the O/E 150.
The CIR 160 includes first to third ports, the first port coupled to the HPA 155, the second port coupled to the antenna 165, and the third port coupled to the LNA 170. The CIR 160 outputs the first and second downstream electrical signals input from the HPA 155 through the first port to the antenna 165 through the second port and outputs first and second upstream electrical signals input from the antenna 165 through the second port to the LNA 170 through the third port.
The antenna 165 transmits an electronic wave corresponding to the first and second downstream electrical signals input from the CIR 160 to the air and generates first and second upstream electrical signals by receiving an electronic wave from the air.
One end of the LNA 170 is coupled to the third port of the CIR 160, and the other end is coupled to the E/O 175. The LNA 170 amplifies each of the first and second upstream electrical signals input from the CIR 160.
One end of the E/O 175 is coupled to the LNA 170, and the other end is coupled to the second optical fiber 135. The E/O 175 electric-to-optical converts the first and second upstream electrical signals input from the LNA 170 to first and second upstream optical signals and outputs the first and second upstream optical signals to the second optical fiber 135.
However, the conventional RoF network 100 described above has problems described below.
Since the CIR 160 has a low separation rate, a case where a portion of the first and second downstream electrical signals input from the HPA 155 through the first port is leaked to the LNA 170 through the third port occurs often. In addition, if an error occurs in impedance matching between the antenna 165 and the CIR 160, a portion of the first and second downstream electrical signals output to the antenna 165 through the second port of the CIR 160 may be reflected by the antenna 165 and re-input to the CIR 160 through the second port of the CIR 160.
In addition, since propagation loss in the air is great, power of the first and second upstream electrical signals generated by the antenna 165 are much lower than power of the leaked or reflected noise signal.
Thus, in the above-described cases, quality of the second upstream electrical signal may be significantly degraded. That is, the leaked or reflected noise signal may saturate the LNA 170 or make the E/O 175 malfunction. In the case of the first upstream electrical signal, since a time slot different from a time slot of the first downstream electrical signal is assigned to the first upstream electrical signal, the first upstream electrical signal is not significantly affected by the leaked or reflected noise signal.